Slide projectors

ABSTRACT

A bowl reflector of modified ellipsoidal shape is suitable for a slide projector, eliminates the need for a condenser and, when used with a flat-coil type filament lamp, enables a substantially uniform light distribution to be obtained. The regions of an ellipsoidal reflector responsible for imaging the cool, dark ends of the filament are effectively omitted from the present reflector.

United States Patent 119 McLintic 1 1 Oct. 30, 1973 SLIDE PROJECTORS1,248,456 12 1917 Clark 353 99 1,502,453 7/1924 Wood 240/4135 R [75]James Mclemuci 1117,0311 7/1928 Ballman et al 353/98 England [73]Assignee: Thorn Lighting Limited, London,

England Primary ExaminerWilliam D. Martin, Jr. [22] Filed: Dec. 1970Att0rney-Joseph C. Ryan and Robert F. O Connell [21] Appl. No.: 98,710

[30] Foreign Application Priority Data [57] ABSTRACT Dec. 17, 1969 GreatBritain 61,558/69 A bowl reflector of modified ellipsoidal shape issuit- 52 US. Cl 353/98, 240/4135 R, 240/103, able for a slide p j i i ts he need for a 350/293 condenser and, when used with a flat-coil typefila: [51] Int. Cl. G03b 21/28 ment p, enables a substantially n m ightdistri- [58] Field of Search 353/98, 99; billion to be Obtained. h rgions of an ellipsoidal re- 240/41.35 R, 103; 350/295, 294, 293 flectorresponsible for imaging the cool, dark ends of the filament areeffectively omitted from the present [56] References Cited reflector-UNITEDSTATES PATENTS Levin 353/98 4 Claims, 5 Drawing Figures Patented'Oct. 30, 1973 3,768,900

2 Sheets-Sheet 1 FIG]. 11 U WILLIAM JAMES McLlNTIC INVENTOR BYWCI MATTORNEY Patented Oct. 30, 1913 3,768,900

2 sheets-sheet 2 WILLIAM JAMES McLINTIC a INVENTOR BY y'xfL ATTORNE YSLIDE PROJECTORS The'present invention relates to improvements relatingto slide projectors.

The optical system of a known slide projector is illustratedschematically in FIG. 1 of the accompanying drawings. Some of the lightproduced by a source 10 is emitted in the direction of a condenser lenssystem 11.

The condenser lens system 11 causes the divergent This lens 12 projectsan image of an illuminated transparency 9 onto a screen (not shown). Thetransparency 9 is located in a gate 13 between the condenser lens system11 and the projector lens 12. A spherical mirror 14 is provided tocollect some of the light produced by the source which is emitted indirections away from the condenser lens system 11. This mirror 14 hasits centre of curvature coincident with the source 10 and reflects lightrays back, through the source 10, towards the condenser lens system 1 1.Light which would otherwise be wasted is therefore utilised. Aconsiderable proportion of the light produced by the source 10 is notcollected by either the mirror 14 or the condenser lens system 12, i.e.,light emitted over the angle 0.

To improve the efficiency of this optical system and thereby increasethe intensity of the light projected, various modifications have beenmade. The angle (1) at the source 10, over which light is collected bythe condenser lens system, may be increased by decreasing the focallength of the condenser lenses and by increasing their diameters i.e.,by increasing their apertures. Furthermore, lenses having deeply curvedsurfaces, of conic rather than spherical configuration, have been usedin attempts to improve the collection angle (1), and hence reduce thecomplementary angle 6. Larger mirrors 14 have been used in conjunctionwith condenser lenses of greater diameter to reduce the angle 0 andthereby reduce the amount of light wasted. A further improvement can beobtained by using compact light sources, which make it possible toreduce the distance between the condenser lens system 11 and the source10 and hence increase the collection angle dz. A particularly attractiveand compact source has a filament produced by winding filament wire ontoa flat mandrel. Unfortunately such a filament is optically dense, andobstructs the light reflected therethrough bythe mirror 14.

It would be desirable to eliminate the use of a condenser lens systemaltogether, both to reduce costs and to reduce the light lost byreflection at each glass-to-air surface. Reflection losses account formore than 8 percent of the light transmitted by each individual lens.

The conventional spherical mirror might be replaced by an ellipsoidalmirror. No condenser lenses would be required because the ellipsoidalmirror collects and can focus the light, produced by a source locatedadjacent one focal point of the mirror, to or adjacent tothe nodal pointof the projector lens. The ellipsoidal mirror may be of the type knownas the cold mirror," in order to minimise the amount of heat generatedbythe source reaching a transparency at the film gate which heat wouldotherwise be reflected towards the transparency. Problems arise withellipsoidal mirrors however especially when used with compact lightsources which have flattened coil filaments. In particular, the lightreflected is non-uniform, so that the illumination of a transparency,and its image projected onto a screen are correspondingly non-uniform.

In accordance with one aspect of the present invention, there isprovided a bowl-shaped reflector of modified ellipsoidal form having areflecting surface composed of two part-ellipsoidal portions symmetricalabout and meeting in a meridional plane of the bowl, the said surfacehaving the shape of the surface of a body formed by parting an ellipsoidor revolution along two planes to form a slice between the two planes,removing the slice and uniting the remaining parts of the ellipsoid attheir surface of parting. Further, in accordance with the inventionthere is provided a screen projector comprising a projection lens, agate and a light source having a coil filament which is located adjacentone focal point of a bowl-shaped reflector of modified ellipsoidalform,- having a reflecting surface composed of two part-ellipsoidalportions symmetrical about and meeting in a meridional plane of thebowl, the said surface having the shape of the surface of a body formedby parting an ellipsoid of revolution along two planes to form a slicebetween the two planes, removing the slice and uniting the remainingparts of the ellipsoid at their surface of parting, the light sourcebeing orientated with the principal axis of its coil filament coincidingwith the said meridional plane and the arrangement being such that theprojection lens is unable to produce an image of the filament itself orof a virtual image of the filament produced by the reflector. Preferablythe reflector has a dichroic reflecting coating which is only partiallyreflecting to heat radiation, so that in use the heat generated by thelight source, which reaches a film or slide located in the gate, isminimised.

The open ends of the hollow coil filament, which may be a flattenedcoil, are cool and dark in comparison with the remainder of the filamentand the ends are not imaged and reflected by the reflector constructionin accordance with this invention. In a true ellipsoidal mirror, thoseareas of its surface which correspond to the part, in effect, absentfrom the present reflector are responsible for reflecting the dark endsof the flattened coil filament and the opening through the filament.This is the reason why a true ellipsoidal mirror used with a flattenedcoil filament light source is unable to provide uniform illumination.

' In practice, the reflector would be manufactured, by

moulding. A master mould for use in producing reflectors according tothe invention may be prepared ini tially by parting a body of trueellipsoidal shape and removing parallel or tapering slices therefromfollowed by joining the remaining parts together to produce a continuousmould surface.

The present invention will now be described by way of example withreference to the remaining figures of the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of the optical system of a prior art slideprojector to which previous reference has been made,

FIG. 2 is an illustration of the virtual image as seen in an ellipsoidalmirror of a flattened coil filament of a light source located therein,

FIG. 3 is an ellipsoidal mirror showing slices of one form which may beremoved to produce a reflector according to the invention,

FIG. 4 is an illustration of the virtual image as seen in a reflectoraccording to the invention having a flattened coil filament light sourcelocated therein, and

FIG. 5 is a schematic drawing of a projector embodying a reflectoraccording to the invention.

In FIG. 2 there is illustrated the virtual image of a flattened coilfilament of a light source as seen in an ellipsoidal mirror when viewedfrom adjacent one focus of the mirror, the filament being locatedadjacent the other focus of the mirror. The mirror has a trueellipsoidally bowl-shaped reflecting surface and has a compact lightsource 21 located on the central axis of the mirror. The light source 21has a flattened coil filament 22 which is manufactured by coiling afilament wire upon a flattened mandrel, and the source 21 is positionedsuch that its filament 22 is centred upon the central axis of the mirror20. A mirror 20 of this shape used with a source 21 incorporating such afilament 22 is unable to reflect the light produced at the filament 22in a uniform manner. As schematically shown, the turns of the filament22 are imaged at 23. Regions 24 on the mirror surface produced images ofthe two ends of the filament coil. In use, the ends of the filament coilare cool relative to the turns themselves, and hence their images aredark in comparison with the images at 23. Thus the intensity of thelight falling onto the mirror surface, and the intensity reflectedtherefrom are non-uniform.

Uniform illuminationcan be obtained if regions 24 are prevented fromimaging the ends of the filament 22. FIG. 3 shows schematically one wayin which a true ellipsoidal mirror 20, for use with a given source 21,may in effect be modified to accomplish this. The drawing shows one endof the filament 22, whose principal plane lies in a meridional plane ofthe ellipsoidal bowl. The shape of the bowl 20 in effect is modified bymaking cuts 31 and 32 through the bowl, the cuts lying in two planessymmetrically disposed to either side of the meridional plane 30 of thebowl. The cuts 31 and 32 define narrow triangular slices 33 whose widthsprogressively increase away from the rim 34 of the bowl 20. The spacingbetween the cuts 31,32 in the region of the ends of the coil 22 isdetermined by the width of the coil, and should be equal to or greaterthan the width of the coil 22. The slices 33 are symmetrical about themeridional plane 30. After removing the slices, the remaining parts areunited by effectively bringing the cut edges 31, 32 together and joiningalong the lines 40, as shown in FIG. 4. This results in a reflectorhaving a continuous surface of modified ellipsoidal bowl-shape. As willbe appreciated, the lines 40 of the joints lie on the meridional plane30.

As will be seen in FIG. 4, the reflector 41 only produces images 23 ofthe turns of the coil 22. The ends of the coil 22 are not imaged by thereflector 41 owing to the absence therefrom of the triangular slices 33.Thus more uniform illumination is reflected by the reflector 41.

An alternative way of achieving the same result is obtained if the cuts31,32, instead of defining narrow tri-' angular slices 33, werearranged, in effect to define parallel slices (not shown). The parallelslices effectively removed should have widths which are governed by thedimensions of the flattened coil filament 22 of the lamp 21. Inparticular, the widths of the parallel slices should be made equal to orlarger than the width of the filament. In use, it is important that thelamp 21 is so orientated that the principal plane of its flattened coilfilament coincides with the plane 30. A reflector having a modifiedellipsoidal bowl shape is produced by joining together the cuts whichdefine the parallel slices.

FIG. 5 shows schematically the optical system of a screen projectorembodying the reflector 41. The reflector 41 has a central aperture 42through which the base 43 of the light source 21 projects for connectionto a power supply. The light source 21 is a compact halogen lamp such asthe Al/223 type. The filament 22 of the source 21 lies in the meridionalplane 30, i.e., in the plane through the line 40. The reflector 41condenses the light received thereby through the gate 44 for a filmtransparency 35 and onto the projection lens.

45. The reflector 41 is so positioned that the angle subtended at thelens 45 by a virtual image (not shown) of the filament 22, produced bythe reflector 41, is substantially the same as the angle subtended atthe lens 45 by the transparency 35 to be projected, when the distancebetween the lens 45 and the transparency 35 is such that a clearlydefined image is projected onto a screen (not shown). The positions andrelative sizes of the gate 44 and filament 22 relative to the lens 45are arranged such that as a result of the depth of focus of the lens,the latter is prevented from producing an image of the filament 22itself and an image of what is reflected by the mirror 41. With thisoptical system, the angle over which the reflector 41 collects light maybe made considerably greater than the collection angle (1) of aconventional projector system.

Since there are no condenser lenses, reduction in manufacturing costs ispossible and light losses by reflection at glass-to-air surfaces arereduced to a minimum.

Although the reflector 41 has been considered as being produced bymodification of true ellipsoidal bowl, in practice the reflector isactually manufactured directly by moulding, followed by the applicationof a reflective coating to its surface. Such a coating is preferably ofa dichroic nature. A master mould can be prepared by parting anellipsoidal body along two parallel or diverging planes and by removingthe resulting parallel or triangular slices 33 embraced thereby, afterwhich the remaining parts are united at their parting surfaces to formthe modified ellipsoidal contour required for moulding the reflector 41.

What is claimed is: 1. A screen projector comprising: a projection lens,a gate, a light source, said light source having a flat-coil filament,and a bowl shaped reflector, said reflector having a reflecting surfacecomprising two segments of ellipsoids of revolution symmetric about andmeeting in a meridional plane of said reflector, the axes of revolutionfor said segments being symmetrically displaced on either side of saidmeridional plane with at least a portion of said meridional plane lyingbetween each segment and its respective axis of revolution, and saidcoil filament being located with the principal axis of said coil lyingin said meridional plane, whereby said reflector is unable to produce animage on the ends of said coil filament. 2. A screen projector accordingto claim 1, wherein said axes of revolution are parallel and spacedapart by a distance at least equal to the width of said filament.

3. A screen projector according to claim 2, wherein said axes ofrevolution diverge from one another with their spacing increasing awayfrom the rim of said reflector, the spacing between said axes ofrevolution in the region of said filament being at least equal to the 5width of said filament.

1. A screen projector comprising: a projection lens, a gate, a lightsource, said light source having a flat-coil filament, and a bowl shapedreflector, said reflector having a reflecting surface comprising twosegments of ellipsoids of revolution symmetric about and meeting in ameridional plane of said reflector, the axes of revolution for saidsegments being symmetrically displaced on either side of said meridionalplane with at least a portion of said meridional plane lying betweeneach segment and its respective axis of revolution, and said coilfilament being located with the principal axis of said coil lying insaid meridional plane, whereby said reflector is unable to produce animage on the ends of said coil filament.
 2. A screen projector accordingto claim 1, wherein said axes of revolution are parallel and spacedapart by a distance at least equal to the width of said filament.
 3. Ascreen projector according to claim 2, wherein said axes of revolutiondiverge from one another with their spacing increasing away from the rimof said reflector, the spacing between said axes of revolution in theregion of said filament being at least equal to the width of saidfilament.
 4. A screen projector according to claim 1, wherein thereflecting surface of said reflector has a dichroic coating which isonly partially reflective to heat radiation.